1. Field of the Invention
The invention relates to improved field distribution in a single element diode laser and in arrays of diode lasers.
2. Description of the Prior Art
Diode laser type of devices, single elements and arrays, emitters and amplifiers, have a long history of continuous developments, many of them documented in patent applications and patents. In this application applicant addresses an improvement that will affect the operation of both single elements and arrays, both emitters and amplifiers.
Most of the single elements are stripe diode lasers, wherein the excited active region seen from the top of the diode laser chip is rectangular in shape. If the active stripe is narrow enough these lasers can operate in a single lateral mode. There are known many structures with stripe geometry. Presently, the most used structures are the etched ridge stripe structures and buried stripe structures. An older type of structure is the gain-guided structure. Among more sophisticated structures reference is made to the lateral ARROW (Anti Resonant Reflecting Optical Wave-guide) structure. If multiple stripes are formed and operate together they form the elements of an array.
All mentioned devices have a lateral structure that comprises a multitude of lateral segments, each of them having similar epitaxial structures.
The ridge structure operates based on a real and positive difference in the effective refractive index in the lateral direction from the ridge-segment to the surrounding segments. Etching part of the surrounding segments top claddings, reduces the effective refractive index in these segments compared with that in non-etched core segment.
The buried structure operates also with a positive difference in the effective refractive index in the lateral direction from the buried segment to the surrounding segments. This difference can be induced by an etch regrowth process. The top cladding in the surrounding segments is etched down and through the active region and the etched material is replaced by regrowth with a material with a lower refractive index than the etched material. When the etching process does not reach the active region a ridge-buried structure is formed.
In the lateral effective refractive index profile, both these structures have a protuberance corresponding to the core segment extent and the radiation is index-guided. In this type of refractive index profile the radiation is confined in the core segment and evanescently decays in the surrounding segments with a lower effective refractive index than that in the core segment. These single element structures are laterally single mode if only the mode with a field distribution with no zero is allowed or it has enhanced favorable conditions for operation. In this case the field distribution in the core segment is almost a half cosine.
The lateral ARROW type of structure confines the radiation into the core segment by reflections from lateral sequences of sub-segments situated in both lateral directions. The first of these sub-segments has the effective refractive index higher than that of the core segment. The second is usually of the same effective refractive index as the core segment. The outside radiation decay is oscillatory and can be controlled by the effective refractive indexed of the lateral segments and by their widths, i.e., by the relation of these widths to each segment lateral wavelength Λi. Normally, each segment i in each lateral sequence has a width equal to an odd multiple of Λi/4. The core segment width is Λs/2. Each lateral sequence acts as an interference cladding. The field distribution in the core segment is a half cosine.
In a coupled array, active segments are coupled and operate together. They form the elements of the array. The segments between elements form the inter-elements. The exact kind of radiation coupling between elements determines a multitude of field distributions that differ from one to another, the super-modes.
To avoid further confusions, it is recalled that the field in any point is a phasor that has amplitude A and phase φ and is described by a complex number E=A exp(iφ). The field intensity I is considered as the square of the field I=|E|2.
There are two super-modes of interest for applications: the in-phase super-mode and the out-of-phase super-mode. The array design can be done to favor only one of these two modes and to discriminate all other modes. The favored mode will be called in this text the “preferred” mode. In the near field, the in-phase mode has all main peaks with the same phase. Due to wavy part of the near field, the far field has one main peak in the direction of the symmetry axis, perpendicular to the array facet, and a few lateral peaks, on both side of the main peak, whose mere existence reduce the brightness of the main peak. The out-of-phase mode has a near field distribution that is closer to a sine function if intensity of all element peaks has equal amplitude. The far field corresponding to such a near field has two symmetric lateral peaks where most of the exit power is concentrated. With external optics one peak can be returned into laser oscillators and amplifiers and all radiation to be emitted into only one, high brightness peak, whose direction is at an angle from the symmetry axis.
There are two types of coupling: evanescent coupling and leaky wave coupling. Arrays with evanescent coupling have multiple protuberances in the effective refractive index profile, one corresponding to each element. These arrays usually operate in the super-mode in which the radiation field in each element is out of phase (also named in anti-phase) with the radiation in adjacent elements, since this super-mode has highest confinement factor and has also the lowest attenuation.
Arrays with leaky wave coupling have protuberances corresponding to each inter-element extent, i.e., the inter-element effective refractive index is higher than element effective refractive index. The radiation distribution in each element can be arranged to be in phase with each other, if inter-element widths are equal to an odd number of its corresponding half lateral wavelength and the element width is half of its corresponding lateral wavelength. Unfortunately the inter-element wavelength is shorter than the element wavelength, due to their differences in effective refractive indexes, and elements are spatially crowded if inter-elements width are only one half wavelength. These arrays had also presented near field patterns that were unequal from element to element, with a cosine shape envelope.
The field distribution in arrays of both types of devices comprises a number of alternating peaks whose intensities usually have a cosine envelope.
The structures for lateral single mode operation mentioned above have the disadvantage that the extension of the field distribution in the active core segment is a half cosine or almost equal to a half cosine. These distributions do not best couple with the shape of the core segment excitation distribution, induced by a current under a contact, excitation distribution that is almost flat under the contact. The inefficient coupling increases the threshold current and reduces the slope efficiency. The threshold current is increased since the overlap of the mode field distribution to the active segments does not have its maximum possible value. The slope efficiency is reduced since, as the Fermi level difference continues to slightly increase after threshold, this extra pumping is not used for power extraction in regions where the field distribution is low. Another disadvantage is that the curvature of the field distribution helps the triggering of the filament formation.
Both types of arrays have the disadvantage that field intensity distributions have a cosine envelope that does not couple well with the uniform excitation distribution from one element to the other. The leaky wave arrays mentioned above have the disadvantage that elements are crowded
One disadvantage of arrays, designed to have as the preferred mode either the in-phase mode or out-of-phase mode is that there are adjacent modes that have relative modal gain close to the modal gain of the preferred mode. If the preferred mode is not discriminated enough, there is a greater possibility for mode switching to non-preferred modes at the beginning or during operation.
The boundary conditions are critical for shaping the field distribution in single element and multiple elements (arrays) diode laser. It is one object of this invention to provide lateral boundary conditions for single element or arrays diode lasers.
It is also an object of this invention is to provide, by design, a single element structure with such boundary conditions that the field distribution in the excited segments is essentially flat. However, for special reasons, by design, the field distribution can be also part of a cosine (convex) or, especially, part of a hyperbolic-cosine (concave) function.
It is another object of this invention to provide arrays with flat field distributions in each element and with equal field amplitudes from element to element.
It is another object of this invention to provide a less restrictive array with array boundaries that assures for the preferred mode that is the out-of-phase mode equal peak field intensity for all elements of array, i. e a flat envelope for the field intensity distribution, but not flat field in each element.